I have a friend that routinely mentions that I can take leftovers and make it into something totally different and flavorful.Considering that I didn't grow up with leftovers I don't know where the talent came from.
Might be because I don't prefer to eat the same thing over and over?
Casseroles using leftovers are easy.Toss protein,carb and veggies(sometimes fruit also) with a binder of cream sauce,tomato sauce,gravy or just a can of soup in to a casserole dish,cover (or not) and bake at 350 for 30 min or until hot in center.
Soups using leftover are easy.Toss leftovers into a pot,add broth,water or juice and call it a day.I will toss small pasta or rice in to extend it if it's not as filling as I would like it to be.
Other...leftovers can go into pastas with any kind of sauce or just butter and seasonings if needed.Or you can toss them into a tortilla wrap or sub bun.OR dump over rice. Or dumb in to eggs and bake,fry, scramble.
My Nonna used to make bean soup on Monday, Tuesday she would add water and mixed veggies(what ever was on hand or in the pantry), Wed she would add rice or small pasta or broken up spaghetti and on Thursday everything was served over stale bread or cube the bread and toss in the soup (bread she had baked the Friday before). It doesn't sound the most exciting meal there was but it was different each week as she changed the type of beans, veggies and whether it was with pasta or rice.The staleness of the bread wasn't noticable with it being in the soup.You could always toast them and make them into croutons for the soup.